Stress-strain behavior of concrete columns confined with hybrid composite materials

A new type of concrete columns was developed at the University of Alabama in Huntsville for new construction to achieve more durable and economical structures. The columns are made of concrete cores encased in a PVC tube reinforced with fiber reinforced polymer (FRP). The PVC tubes are externally reinforced with continuous impregnated fibers in the form of hoops at different spacings. The PVC acts as formwork and a protective jacket, while the FRP hoops provide confinement to the concrete so that the ultimate compressive strength and ductility of concrete columns can be significantly increased. The volume of fibers used in this hybrid column system is very modest compared to other existing confinement methods such as FRP tubes and FRP jackets. This paper discusses the stress-strain behavior of these new composite concrete cylinders under axial compression loading. Test variables include the type of fiber, volume of fiber, and the spacing between the FRP hoops. A theoretical analysis was performed to predict the ultimate strength, failure strain and the entire stress-strain curve of concrete confined with PVC-FRP tubes. Test results show that the external confinement of concrete columns by PVC-FRP tubes results in enhancing compressive strength, ductility and energy absorption capacity. A comparison between experimental and analytical results indicates that the models provide satisfactory predictions of ultimate compressive strength, failure strain and stress-strain response.     

Dynamic compressive and splitting tensile properties of concrete containing recycled tyre rubber under high strain rates

In order to raise the efficiency of resource utilization, recycling waste rubber particles into concrete as aggregate has been widely accepted. When the size and content of the rubber particles are appropriate, rubberized concrete can achieve many excellent properties. This study investigated the impact of rubber replacement on dynamic compressive and splitting tensile properties of concrete. The split Hopkinson pressure bar tests of rubberized concrete containing 5%, 10%, 15% and 20% volume replacement for sand were completed. The failure modes, stress curves and dynamic strength values of rubberized concrete under high strain rates were recorded. The results reveal that the dynamic compressive and splitting tensile strength of rubberized concrete decrease with increasing rubber content. Meanwhile, peak strain increases with increasing rubber content. Dynamic increase factors (DIFs) of compressive and splitting tensile strength also were calculated, where rubberized concrete shows a stronger strain rate sensitivity. The analysis of specific energy absorption illustrates that rubberized concrete with 15% rubber replacement has the best impact toughness. In addition, ratios of dynamic compressive–tensile strength of rubberized concrete were calculated, which are between 3.82 and 5.39.     

Experimental study of FRP tube encased concrete cylinders exposed to fire

In recent years, fiber-reinforced polymer (FRP) tube encased concrete cylinders have been widely studied for their use in civil infrastructure. However, the performance of these FRP/concrete systems exposed to fire is a serious concern due to the combustibility of FRPs. This paper presents test results of temperature distributions and residual strength of three groups of FRP tube encased concrete cylinders (Groups 1, 2, and 3) exposed to a jet fire of 982 °C for a time period of 4 min, 8 min, and 12 min, respectively. The temperature distribution of the confined concrete cylinders under fire was recorded by the embedded thermocouples. After fire test, uniaxial compression tests were conducted to evaluate the residual strength. The results were compared with that from a group of FRP tube encased concrete cylinders without fire exposure (Group 4). To improve the fire resistance of the confined concrete cylinders, a fireproof additive was also applied to three other groups of specimens (Groups 5, 6, and 7) and these specimens were also fire tested for 4 min, 8 min, and 12 min, respectively. SEM was used to examine the burn-out of resin by fire. The test results show that the fire exposure has a significant effect on reducing the residual strength of the confined concrete cylinders. The resin has been fully burned-out and carbonized after 8 min of fire exposure. After 12 min of burning, the FRP tube basically loses its capacity to provide any meaningful confinement. The fireproof additive used is effective in minimizing the effect of the fire hazard.     

Harsh environments effects on the axial behaviour of circular concrete-filled fibre reinforced-polymer (FRP) tubes

Concrete-filled fibre-reinforced polymer (FRP) tubes (CFFTs) are becoming an attractive system for structural elements proposed to harsh environments. FRP tube provides a corrosion resistant element, reinforcement, confinement for the concrete core, and a stay-in-place formwork. Harsh environments may affect the mechanical performance of the FRP tube, which consequently affect the structural response of the CFFT members. This project investigates the environmental degradation and the durability of concrete cylinders unconfined and confined by filament-wound glass-FRP tubes. Standard plain concrete cylinders and CFFT cylinders were immersed in pure water, salt and alkaline solutions, and exposed to 200 freeze–thaw cycles, between −40 °C and +40 °C. Then, the cylinders were tested under uniaxial compression test to evaluate their performance by comparing the stress–strain behaviour and their ultimate load capacities. Test results indicated that the FRP tube, in CFFTs, is significantly qualified as a sustainable coating material to resist the harsh environments attacks. Theoretical predictions using long term confinement models from CSA and ACI codes are presented.     

Effect of fiber orientation on the structural behavior of FRP wrapped concrete cylinders

In this study, 27 concrete cylinders with a diameter of 152.4 and a height of 304.8 mm were prepared. Among them, 18 cylinders were wrapped using two layers of fiber reinforced polymer (FRP) with six fiber orientations; six cylinders were wrapped using four layers of FRP with fibers in axial or hoop direction only; the remaining three cylinders were used as control. The FRP used was E-glass fiber reinforced ultraviolet (UV) curing vinyl ester. Fifteen coupon specimens were prepared to experimentally determine the tensile strength of the FRP with fibers oriented at 0°, 45°, and 90° from the loading direction. Co-axial compression tests were conducted on the wrapped cylinders and control cylinders. The test results were compared with existing confinement models. It is found that the strength, ductility, and failure mode of FRP wrapped concrete cylinders depend on the fiber orientation and wall thickness. Fibers oriented at a certain angle in between the hoop direction and axial direction may result in strength lower than fibers along hoop or axial direction. A larger database is desired in order to refine the existing design-oriented confinement models.     

Effects of wrap thickness and ply configuration on composite-confined concrete cylinders

The behavior of small-scale fiber reinforced polymer (FRP) wrapped concrete cylinders under uniaxial compressive loading was investigated through nonlinear finite element analysis. Two parameters were considered for this numerical study: the FRP wrap thickness, and the ply configuration. Performances of numerical models with “hoop-angle-hoop” and “angle-hoop-angle” ply configurations were compared, where the terms “hoop” and “angle” indicate that wraps were oriented at an angle of 0° and 45° in reference to circumferential direction, respectively. The finite element analysis results showed substantial increase in the axial compressive strength and ductility of the FRP confined concrete cylinders as compared to the unconfined ones. The cylinders with “hoop-angle-hoop” ply configuration in general exhibited higher axial stress and strain capacities as compared to the cylinders with the “angle-hoop-angle” ply configuration. The increase in wrap thickness also resulted in enhancement of axial strength and ductility of the concrete cylinders.     

Strength development and chloride penetration in rubberized concretes with and without silica fume

The study presented herein has been carried out in order to investigate the strength development and chloride permeability characteristics of plain and rubberized concretes with and without silica fume. For this purpose, two types of tire rubber, namely crumb rubber and tire chips, were used as fine and coarse aggregate, respectively, in the production of rubberized concrete mixtures which were obtained by partially replacing the aggregate with rubber. Two water-cementitious material (w/cm) ratios (0.60 and 0.40), three moist curing periods (3, 7, and 28 days), four designated rubber contents (0, 5, 15, and 25 by total aggregate volume), two silica fume content (0 and 10% by weight of cement), and five different testing ages (3, 7, 28, 56, and 90 days) were considered as experimental parameters. The results indicated that for a given w/cm ratio and moist curing period, the use of rubber significantly aggravated the chloride ion penetration through concrete but the degree of the rate of the increment of the chloride permeability depended on the amount of the rubber used. When the curing period was extended from 3 to 28 days, the reduction in the magnitude of chloride penetration depth was notably higher for the rubberized concretes, even at a rubber content of as high as 25%. It was also observed that silica fume may be considered as a remedy to enhance the chloride penetration resistance of the rubberized concretes.     

Effects of partial oxidation of crumb rubber on properties of rubberized mortar

It is a generally accepted result that the inclusion of rubber particles causes the concrete to degrade physical properties of the concrete. In this study the crumb rubber was partially oxidized and used as additives of mortars. Dramatically, the compressive strength of the rubberized mortars (with 6 wt.%) was greater than that of mortars without the crumb rubber. To understand these phenomena, Fourier Transform Infrared Spectroscopy (FTIR) was used to explore the functional groups on surfaces of the crumb rubber, and Scanning Electron Microscopy (SEM) was used to observe microstructures of mortars. With the partial oxidation treatment, rubber surfaces produced hydrophilic functional groups as indicated by IR spectra and the hydration of the cement near the crumb rubber was enhanced as shown by SEM, leading to stronger mortars.     

Mussel Inspired Modification of Rubber Crumbs for Improved Interfacial Adhesion in Rubber Cement Mortar

Crumb rubbers exhibit extensive potential applications as infrastructure materials due to the low elastic modulus. Nevertheless, the poor interfacial adhesion between rubber crumb and cement matrix limits the scale applications of crumb rubbers in cement-based composites. In this study, mussel-inspired modification of crumb rubbers is investigated. The hydrophilicity of rubber surface has apparently improved after polydopamine (PDA) modification. Effects of the surface modifications of rubbers on the compressive strength, fluidity, and tribology behaviors of rubberized mortars have been systematically characterized. The superiority of PDA modification for crumb rubbers has been demonstrated by comparing those with the other polyphenol modifications and the routine oxidation modification. The compressive strength of the PDA modified rubber cement mortar increases by 37% comparing with that of the ordinary rubber cement mortar. The mechanical and low-temperature tribology behaviors of PDA-rubberized mortars indicate a promising way to improve the service performance of the rubberized mortars and concretes.

     

Substandard reinforced concrete members subjected to compression: FRP confining effects

The investigation focuses on the effectiveness of fiber-reinforced polymer (FRP) confinement in upgrading ductility and strength of reinforced concrete members under axial monotonic compression. An experimental program is presented that extends available database to address the behavior of old type members with square section, having extremely low concrete strength and potential longitudinal bars’ premature buckling. Reinforced concrete specimens were strengthened by carbon or glass FRP wraps while plain FRP confined concrete specimens were also constructed and tested to evaluate comparatively the confining effects of steel stirrups, FRP wraps, or of dual confinement. The achieved strength, ductility and energy absorption levels of the specimens were quantified to assess the effect of the longitudinal bars. Finally, a handy design-oriented empirical strength model is proposed. According to the proposed approach, no estimation of effective stress or strain at failure of FRP jacket is necessary. The satisfactory accuracy of the predictions of the proposed model is demonstrated through comparison against existing models and over a large database of results on uniform confinement as well as over presented specimens.     

Influence of concrete strength and confinement method on axial compressive behavior of FRP confined high- and ultra high-strength concrete

This paper presents an experimental investigation on the effect of concrete compressive strength and confinement method on confined high and ultra high-strength concrete (HSC and UHSC) specimens. A total of 55 fiber reinforced polymer (FRP) confined concrete specimens were tested under monotonic axial compression. All specimens were cylinders with 152 mm diameter and 305 mm height and confined by carbon FRP (CFRP). Three different concrete mixes were examined, with average compressive strengths of 35, 65 and 100 MPa. The effect of the confinement method was also examined with FRP-wrapped specimens compared to FRP tube-encased specimens. Axial and lateral behavior was recorded to observe the axial stress–strain relationship and lateral strain behavior for concentric compression. Ultimate axial and lateral conditions are tabulated and the complete stress–strain curves have been provided. The experimental results presented in this paper provide a performance comparison between FRP-confined conventional normal-strength concrete (NSC) and the lesser understood area of FRP-confined HSC and UHSC. The results of this experimental study clearly indicate that above a certain confinement threshold, FRP-confined HSC and UHSC exhibits highly ductile behavior, however for the same normalized confinement pressures, axial performance of FRP-confined concrete reduces as concrete strength increases. The results also indicate that ultimate conditions of FRP-wrapped specimens are similar to those confined by FRP tubes, however a performance difference is evident at the transition region. The performance of 10 existing stress–strain models were assessed against the experimental datasets and the performance of these models discussed. The results of this model assessment revealed the need for further development for stress–strain models developed specifically for FRP-confined HSC or UHSC.     

等强条件下橡胶粉对碾压混凝土强度与变形性能的影响

采用橡胶粉等体积代砂的方法,选择橡胶粉体积含量为5%、10%和12 %,通过试验探讨了等抗压强度(40 MPa)条件下橡胶粉用量对碾压混凝土力学强度、抗裂性能和变形性能的影响及其变化规律。结果表明：橡胶粉的掺入使碾压混凝土的韧性得到改善,压缩破坏表现出明显的韧性破坏特征,碾压橡胶混凝土的立方体抗压强度、轴心抗压强度、抗拉强度和抗弯强度之间具有与普通混凝土相近的对应关系;等抗压强度条件下,碾压橡胶混凝土的抗弯强度、抗拉强度和极限拉伸值较不掺橡胶粉的碾压混凝土有所增大;橡胶粉的添加不会改善碾压混凝土的干缩性能,反而会使干缩变形速率和干缩变形量有所增大。     

Measurement and prediction of the strength of rubberized concrete

Illegally discarded piles of automobile tires are sources of potential hazards. Current disposal methods are wasteful and costly as they require either consumption of landfill space or continuous costly maintenance. A solution to the problem of scrap-tire disposal is the potential use of tire chips and crumb rubber as mineral aggregate substitutes in Portland cement concrete mixes. In this study, some of the engineering properties of rubberized concrete were examined and a neural network was developed to predict its compressive and tensile strengths. Rubberized concrete was found to possess good esthetics, acceptable workability, and a smaller unit weight than normal concrete. However, rubberized concrete did not perform as well as normal concrete under repeated freeze-thaw cycles. It exhibited lower compressive and tensile strength than that of normal concrete. Unlike normal concrete, rubberized concrete had the ability to absorb a large amount of plastic energy under compressive and tensile loads. It did not demonstrate the typical brittle failure, but rather a ductile, plastic failure mode. Test results were analyzed so that a model can be developed to predict the strength of rubberized concrete. Two neural network models were developed to predict the reduction in the compressive and tensile strength as a result of replacing mineral aggregate with rubber aggregate. A maximum difference of 9·2% between test results and model prediction was detected during the testing of the neural networks.     

玄武岩纤维布约束高温损伤混凝土方柱轴压力学性能试验

对36个玄武岩纤维布增强聚合物基复合材料（BFRP）约束的高温损伤混凝土方柱和15个不同高温损伤的对比试件进行了轴压试验。试验表明,玄武岩纤维布横向约束能改变高温损伤后混凝土方柱的破坏形态,显著提高混凝土方柱的轴压强度和变形能力。其中三层玄武岩纤维布包裹的200℃、400℃、600℃和800℃高温损伤混凝土方柱轴压强度分别提高了48%、130%、206%和389%,轴向变形分别提高了433%、344%、319%和251%。采用典型的纤维增强聚合物基复合材料（FRP）约束常温未损伤混凝土轴压力学性能的设计模型预测FRP约束高温损伤混凝土的轴压强度和变形时存在较大的偏差。通过构建柱状膜结构静水压力平衡模型和约束混凝土方柱与FRP体积应变能平衡模型,分别改进了FRP约束混凝土方柱轴压极限应力和极限应变计算模型的基本形式。基于该基本形式和试验数据,分别确定了BFRP约束高温损伤混凝土方柱轴压极限应力和极限应变计算中与温度相关的参量,提出了适用于高温损伤混凝土方柱的轴压极限应力和极限应变的设计模型。      

Effect of confinement level,aspect ratio and concrete strength on the cyclic stress–strain behavior of FRP-confined concrete prisms

Behavior of rectangular concrete columns confined with FRP composites depends on several parameters, including unconfined concrete strength, confinement level, aspect ratio of cross-section (defined as the depth/width of the cross-section), and the sharpness of the section corners. For modeling the cyclic stress–strain behavior of FRP-confined rectangular concrete columns, effect of column parameters on the cyclic behavior of these columns should be examined properly. In this paper, effects of unconfined concrete strength, confinement level and the aspect ratio of cross-section are studied. The test database includes 10 prisms from recent study of authors and 18 prisms from a new experiment. Results of tests show that some aspects of cyclic behavior of FRP-confined concrete prisms such as envelope curve and stress deterioration are unaffected by the considered parameters. Results also indicate that the plastic strain decreases as the unconfined concrete strength increases, but it is independent of the aspect ratio and the confinement level. While the reloading path in all specimens was almost linear, the unloading path was highly nonlinear and was affected by unconfined concrete strength.     

氨化与磺化改性橡胶混凝土机理及强度研究

为改善橡胶混凝土的强度,本工作选用尿素和NaHSO3两种改性剂对橡胶颗粒进行氨化与磺化改性。借助傅里叶红外光谱仪验证引入的极性亲水基团,利用水接触角试验仪分析改性橡胶的亲水性能,通过粘接强度试验和抗压强度试验研究改性橡胶对橡胶混凝土强度的提升效果,用扫描电子显微镜(SEM)表征橡胶混凝土的微观破坏形貌。结果表明:氨化改性在橡胶颗粒表面引入羰基和氨基等,磺化改性在橡胶颗粒表面引入羟基和磺酸基等;与普通组相比,氨化与磺化改性后,改性橡胶-水接触角分别降低31°、35°,改性橡胶与水泥净浆的粘接强度分别提升44%、53%,改性橡胶混凝土的抗压强度在不同橡胶粒径与掺量条件下均得到提升;SEM结果表明极性亲水基团的引入能较好地提升橡胶颗粒与胶凝材料的粘接性能,有利于改善两者结合界面薄弱的问题,增强橡胶混凝土的整体强度。     

New formulation for compressive strength of CFRP confined concrete cylinders using linear genetic programming

This paper proposes a new approach for the formulation of compressive strength of carbon fiber reinforced plastic (CFRP) confined concrete cylinders using a promising variant of genetic programming (GP) namely, linear genetic programming (LGP). The LGP-based models are constructed using two different sets of input data. The first set of inputs comprises diameter of concrete cylinder, unconfined concrete strength, tensile strength of CFRP laminate and total thickness of utilized CFRP layers. The second set includes unconfined concrete strength and ultimate confinement pressure which are the most widely used parameters in the CFRP confinement existing models. The models are developed based on experimental results collected from the available literature. The results demonstrate that the LGP-based formulas are able to predict the ultimate compressive strength of concrete cylinders with an acceptable level of accuracy. The LGP results are also compared with several CFRP confinement models presented in the literature and found to be more accurate in nearly all of the cases. Moreover, the formulas evolved by LGP are quite short and simple and seem to be practical for use. A subsequent parametric study is also carried out and the trends of the results have been confirmed via some previous laboratory studies.     

Waste tire fiber modified concrete

It is well known that waste tire chip modified concrete has very high toughness. However, its strength and stiffness are reduced significantly. Sometimes, the strength and stiffness are so low that the modified concrete cannot be used as structural materials. In this paper, waste tires were used in the form of fibers and waste tire fiber modified concrete was developed. A total of 42 ∅ 152.4 mm by 304.8 mm (6 in. by 12 in.) cylinders were prepared to conduct compressive strength, split tensile strength, and modulus of elasticity tests. Among them, six cylinders were prepared without waste tires, and six samples were prepared using waste tire chips. They were used as controls in this study. The remaining 30 samples were prepared using waste tire fibers with various lengths and stiffness. The test results show that, while the strength and stiffness of the concrete modified with waste tire fibers were still lower than those without waste tires, they were higher than those with waste tire chips. A two-phase composite model was also developed and a finite element analysis was conducted on this model. The analysis results show that the stress concentration in waste tire fiber modified concrete is lower than that in waste tire chip modified concrete. Using stiffer and thinner waste tire fibers can further reduce stress concentrations.     

钢纤维-橡胶/混凝土单轴受压全曲线试验及本构模型

将钢纤维(SF)掺入橡胶混凝土中,能够改善由于橡胶颗粒掺入导致的强度降低,并进一步增加延性。为研究SF-橡胶/混凝土的抗压性能,配制得到SF体积分数分别为0vol%、0.5vol%、1.0vol%和1.5vol%及橡胶颗粒等体积替换砂率为0%、10%和20%的10组SF-橡胶/混凝土试件,并进行单轴受压全曲线试验。结果表明:SF的桥联作用及其与橡胶颗粒的协同作用可改善混凝土的抗压性能,试件破坏呈明显延性特征。随SF掺量的增加,SF-橡胶/混凝土试件的抗压强度及弹性模量均明显增大,其相应峰值应力的应变及全曲线峰值后延性也相应增加;随橡胶颗粒掺量的增加,SF-橡胶/混凝土试件相应峰值应力的应变及全曲线峰值后延性增加,而抗压强度及弹性模量有所减小。在已有研究基础上,通过曲线拟合试验数据,提出适用于SF-橡胶/混凝土的单轴受压应力-应变全曲线数学表达式,模型与试验结果吻合较好,为此类混凝土的结构分析设计提供了理论基础。      

Performance of externally FRP reinforced columns for changes in angle and thickness of the wrap and concrete strength

In this study, the performance of axially loaded, small-scale, and fiber-reinforced polymer (FRP) wrapped concrete columns with various wrap angle configurations, wrap thicknesses, and concrete strengths was investigated through nonlinear finite element analysis. Three different wrap thicknesses, wrap ply angle configurations of 0°, ±15°, and 0°/±15°/0° with respect to the circumferential direction, and concrete strength values ranging from 3 ksi to 6 ksi were considered. An existing experimental study on FRP-confined circular columns in the literature was utilized to validate numerical analysis models. The finite element analysis results showed substantial increase in the axial compressive strength and ductility of the FRP-confined concrete cylinders as compared to the unconfined cylinders. The increase in wrap thickness also resulted in enhancement of axial strength and ductility of the concrete columns. The gain in axial compressive strength in FRP-wrapped concrete columns was observed to be higher for lower strength concrete and the highest in the columns wrapped with the 0° ply angle configuration.